1. Field of the Invention
The present invention relates to a method for improving efficiencies in the cleaning processes of finely-divided carbonaceous material and specifically to a method for improving flotation, dewatering and reconstitution in fine coal processing with the addition of a single additive at the beginning of the process.
2. Background of the Invention
Demand for environmentally acceptable coal continues to increase. This results in the need for improvements in physical coal cleaning processes. Classical coal beneficiation involves separation of the combustible and mineral matter of coal by methods based on differences in density. However, mechanized coal mining techniques, combined with the need to liberate mineral matter through deeper cleaning, has lead to the industry having to deal with treating larger amounts of coal fines. To optimize such mineral matter rejection, coal is reduced to sizes smaller than 28 mesh (600 microns (.mu.m)). This emphasis on fine coal beneficiation has lead to separation processes that depend on differences in surface properties of the particles rather than on their densities.
Most conventional fine coal cleaning processes employ water or water-based media for the removal of pyritic sulfur and ash-forming mineral matter from raw coal before sale. However, small particle size distribution of these product slurries makes subsequent dewatering of these fine coal products a difficult problem. Most techniques require application of expensive and time consuming thermal dryers. In addition, the thermally dewatered product, owing to its dusty nature and its increased reaction rate with oxygen, possesses its own set of handling, transportation and storage problems, and it often causes safety and environmental problems. Some of these problems include spontaneous combustion, explosion, wind erosion, and dust pollution.
The rejection of water from fine coal particles by conventional vacuum filtration and centrifugation processes is enhanced by the addition of surfactants and flocculants. A commercial water-based (oil-in-water) asphalt emulsion has been used for the dewatering and reconstitution of fine coal particles. (U.S. Pat. No. 4,969,928). However, these asphalt emulsions, prepared with cationic type surfactants, are not collectors for the initial coal cleaning step, which is coal flotation. Emulsified asphalt also fails to provide adequate dewatering and dust reduction when slurry temperature is low. Furthermore, asphalt is a product of the petroleum refining process, and not naturally formed, thereby leading to high costs associated with its use.
A cost effective fine coal beneficiation process is needed to separate coal fines from mineral matter, dewater the clean coal, and then reconstitute the clean coal into a low moisture and low dustiness product for utility use. The process should embody a single addition step wherein emulsions of heavy hydrocarbons are used as surface selective additives to enhance flotation, dewatering and agglomeration of fine coal products.